Various systems including a server and a thin client which transmit/receive an image have been proposed (for example, see Patent Literature 1). As a server that transmits an image to the thin client, for example, there has been proposed a server that encodes and transmits image data of a rectangular region including an updated region of an image (for example, see Patent Literature 2). According to such a transmission scheme, when only a part of an image has been updated, it is sufficient if the server encodes only image data of some regions and transmits the encoded image data to the thin client. Accordingly, it is possible to reduce the amount of transmission data.
Furthermore, when the server transmits image data to the thin client via a transmission network, there is a case in which the server encodes the image data in order to compress a data amount (for example, see Patent Literature 3). When the server uses irreversible conversion such as quantization at the time of data compression, there is a case in which the quality of a decoded image at a reception side is degraded as compared with the original image of a transmission side. Furthermore, even when data deficiency or a transmission error has occurred in a transmission path and the like, there is a case in which image quality of a received image is degraded. In order to prevent the degradation of the image quality from being transmitted to a subsequent frame, there has been proposed a scheme in which the server regularly inserts an intra update (intra refresh) signal into an image and transmits the image (for example, see Patent Literature 4). Herein, when an entire frame is encoded, a code amount is significantly increased. Therefore, in general, in the case of performing intra update, the server divides a frame into a plurality of partial regions (tiles) and transmits the tiles little by little.
For example, the device disclosed in Patent Literature 4 transmits a code of an update region and then performs the intra update in the range of a surplus band while making a round of a low quality tile. Therefore, even when an area of an update region of a frame has been largely changed, an area of a region to be subjected to intra update is changed, so that it is possible to achieve high quality while sufficiently utilizing the surplus band. For example, in a frame not updated from a previous frame or a frame in which an area of an update region is small, the server can perform intra update with respect to many tiles as far as they are included in the surplus band. Furthermore, the server preferentially performs intra update for a tile of a region encoded with low quality or a tile not updated for a long time, so that it is possible to shorten a period in which a low quality tile remains in an image.
Furthermore, Patent Literature 5 discloses an image transmission system that determines a transmission priority for each update region and transmits an update region in a priority order.
Hereinafter, a general encoding process will be described using a detailed example. FIG. 9 illustrates an example of a frame in which some regions have been updated. When a frame 91 and a frame (not illustrated) prior to the frame 91 have been compared with each other, it is assumed that regions A to C have been updated in the frame 91. It is sufficient if the server encodes only the regions A to C and transmits the frame 91 to the thin client. Herein, the server divides a communication band by a target frame rate and calculates a target code amount. Furthermore, on the basis of a data amount before compression of the update regions A to C and the target code amount, the server calculates a target compression rate. For example, it is assumed that a communication band is 3000 kbps (kbit per second) and the target frame rate is 30 fps (frames per second). Then, the server obtains 100 kbit/frame as the target code amount. Furthermore, it is assumed that the data amount before compression of the update regions A to C is 1402 kbit. Then, the server obtains about 1/14 as the target compression rate. The server compresses image data of the update regions A to C at the target compression rate and transmits the compressed image data to the thin client.
Furthermore, general intra update will be described using a detailed example. FIG. 10 is a schematic diagram illustrating an example of a change in the state of continuous frames. One frame has been divided into tiles 95 with a predetermined size. It is assumed that a region 96 has been updated in a frame 1 and a region 97 has been updated in a next frame 2. Furthermore, in the present example, the quality of a place, where update has occurred, is assumed to be degraded in order to simplify a description. In FIG. 10, tiles with degraded quality are represented with a dotted pattern. Furthermore, tiles with high quality are represented with a white color. The server encodes each tile related to the region 96 of the frame 1 and transmits the encoded tiles to the thin client. Similarly, in relation to the next frame 2, the server encodes each tile related to the region 97 and transmits the encoded tiles to the thin client. At the time of transmission of the frames 1 and 2, it is assumed that a surplus band has not existed.
In subsequent frames 3 to 16, it is assumed that an update region has not been generated. Then, the server does not perform encoding and transmission of an update region in relation to the frames 3 to 16. The server performs intra update in a surplus band generated by performing no encoding and transmission. FIG. 10 illustrates the case in which the server performs intra update in one frame by one tile. As illustrated in FIG. 10, the server makes a round of tiles with degraded quality for the frames 3 to 16 and performs intra update. As a consequence, in the frame 16, the number of tiles with degraded quality is reduced as compared with the frame 2. In the example illustrated in FIG. 10, the priority of the intra update of the tiles with degraded quality becomes high toward tiles positioned at an upper end of a screen. Furthermore, in tiles of the same stage, the priority becomes high toward left tiles when viewed from a user.